GMR Biosensor with Enhanced Sensitivity

ABSTRACT

A sensor array comprising a series connection of parallel GMR sensor stripes provides a sensitive mechanism for detecting the presence of magnetized particles bonded to biological molecules that are affixed to a substrate. The adverse effect of hysteresis on the maintenance of a stable bias point for the magnetic moment of the sensor free layer is eliminated by a combination of biasing the sensor along its longitudinal direction rather than the usual transverse direction and by using the overcoat stress and magnetostriction of magnetic layers to create a compensatory transverse magnetic anisotropy. By making the spaces between the stripes narrower than the dimension of the magnetized particle and by making the width of the stripes equal to the dimension of the particle, the sensitivity of the sensor array is enhanced.

This is a Divisional Application of U.S. patent application Ser. No.11/497,162, filed on Aug. 1, 2006, which is herein incorporated byreference in its entirety and assigned to a common assignee.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the detection of small magnetized particles(beads) by a GMR sensor, particularly when such particles or beads areattached to molecules whose presence or absence is to be determined in achemical or biological assay.

2. Description of the Related Art

GMR (giant magnetoresistive) devices have been proposed as effectivesensors to detect the presence of specific chemical and biologicalmolecules (the “target molecules”) when, for example, such targetmolecules are a part of a fluid mixture that includes other moleculeswhose detection is not necessarily of interest. The basic methodunderlying such magnetic detection of molecules first requires theattachment of small magnetic (or magnetizable) particles (also denoted“beads”) to all the molecules in the mixture that contains the targetmolecules. The magnetic beads are made to attach to the molecules bycoating the beads with a chemical or biological species that binds tothe molecules in the mixture. Then, a surface (i.e., a solid substrate)is provided on which there has been affixed receptor sites (specificmolecules) to which only the target molecules will bond. After themixture has been in contact with the surface so that the targetmolecules have bonded, the surface can be flushed in some manner toremove all unbonded molecules. Because the bonded target molecules (aswell as others that have been flushed away) are equipped with theattached magnetic beads, it is only necessary to detect the magneticbeads to be able, at the same time, to assess the number of capturedtarget molecules. Thus, the magnetic beads are simply “flags,” which canbe easily detected (and counted) once the target molecules have beencaptured by chemical bonding to the receptor sites on the surface. Theissue, then, is to provide an effective method of detecting the smallmagnetic beads, since the detection of the beads is tantamount todetection of the target molecules.

One prior art method of detecting small magnetic beads affixed tomolecules bonded to receptor sites is to position a GMR device beneaththem; for example, to position it beneath the substrate surface on whichthe receptor sites have been placed.

FIG. 1 is a highly schematic diagram (typical of the prior artmethodology) showing a magnetic bead (10) covered with receptor sites(20) that are specific to bonding with a target molecule (30) (shownshaded) which has already bonded to one of the sites. A substrate (40)is covered with receptor sites (50) that are also specific to targetmolecule (30) and those sites may, in general, be different from thesites that bond the magnetic particle to the molecule. The targetmolecule (30) is shown bonded to one of the receptor sites (50) on thesurface.

A prior art GMR sensor (60), shown without any detail, is positionedbeneath the receptor site. As shown schematically in the cross-sectionalview of FIG. 2 a, the prior art GMR sensor ((60) in FIG. 1) (60), ispreferably in the form of a laminated thin film stripe that includes amagnetically free layer (610) and a magnetically pinned layer (620)separated by a thin, non-magnetic but electrically conducting layer(530). Typically, the sensor will also include a capping layer oroverlayer (550). The GMR properties of such a film stripe causes it toact essentially as a resistor whose resistance depends on the relativeorientation of the magnetic moments of the free and pinned layers (shownhere as arrows (640), (650) both directed out of the figure plane). FIG.2 b shows, schematically, an overhead view of the sensor of FIG. 2 a,showing more clearly the direction of the magnetic moments (640) and(650), shown dashed, as it is below (640). Also shown are two lobesoutlining a region of equal strength of an external magnetic field B(800). The field strength is shown directionally as arrows (160) thatwould be produced by a magnetized particle (not shown here) that ispositioned above the sensor, as shown in FIG. 1. This will also be shownmore clearly in FIG. 3, below. The field of the lobes will deflect (640)(deflection not shown), but leave (650) unchanged.

FIG. 3 shows, schematically, a magnetic particle (10) situated (bybinding) over a surface layer (45) formed on a substrate (40) in atypical prior art configuration. The surface layer is required tosupport the bonding sites and can be a layer of Si₃N₄ and the substratecan be a Si substrate on or within which the required circuitry can beformed. For simplicity, a target molecule is not shown. A GMR sensor(60), as illustrated in any of the previous figures, is positionedbetween the surface layer and the substrate (40). An electromagnet (100)is positioned beneath the substrate and creates a magnetic field H (120)directed vertically through the substrate, the GMR sensor, and themagnetic bead. The external field, H, is directed perpendicularly to themagnetic moments of the GMR sensor (640), (650) so as not to changetheir relative orientations. Because of the magnetic properties of thebead, the external field H (120), induces a magnetic moment M (150),shown as an arrow in the bead which, in turn, produces a magnetic fieldB (160) that extends beyond the boundary of the bead as shown by thedashed lobes. The magnetic field B (160), in turn, penetrates the planeof the sensor and its component in that plane (shown as the lobes (800)in FIG. 2 b) can change the orientation of the magnetic moment of thesensor free layer as is shown schematically in FIGS. 4 a and 4 b.

The magnetization of the free layer (640), is now changed in directionrelative to the magnetization of the pinned layer (650), because of thepresence of the magnetic field of the magnetized bead (160) that isdirected within the plane of the free layer. Because the presence of themagnetized bead affects the magnetic moment of the free layer, itthereby, changes the resistance of the GMR sensor strip. By detectingthe changes in resistance, the presence or absence of a magnetized beadis made known and, consequently, the binding of a target molecule isdetected. Ultimately, an array of sensors can be formed beneath asubstrate of large area that is covered by a large number of bindingsites. The variation of the resistance of the sensor array is then agood indication of the number of target molecules that has been capturedat sites and that number, in turn, can be related to the density of suchtarget molecules in the mixture being assayed.

As is well known by those skilled in the field, although themagnetization of the free layer moves in response to external magneticstimuli during operation of the sensor, the magnetization of the freelayer is preferably fixed when the sensor is in a quiescent mode and notacted on by external fields. The fixing of the free layer magnetizationunder these conditions is called “biasing” the free layer and theposition of the magnetic moment of the free layer in this position iscalled its bias point. It is also known to those skilled in the art thatthe bias position of the free layer is subject to the effects ofhysteresis, which means that the bias position is not maintained afterthe magnetization of the free layer is made to cycle through positiveand negative directions by external magnetic stimuli and a quiescentstate is once again achieved. This hysteresis has a negative impact onthe reproducibility of sensor readings, particularly when the externalstimuli moving the free layer magnetization are small to begin with. Oneof the objects of the present invention will be to eliminate the adverseeffects of hysteresis. Given the increasing interest in theidentification of biological molecules it is to be expected that thereis a significant amount of prior art directed at the use of GMR sensors(and other magnetic sensors) to provide this identification. A detailedresearch paper that presents an overview of several different approachesas well as the use of GMR sensors is: “Design and performance of GMRsensors for the detection of magnetic microbeads in biosensors” J. C.Rife et al., Sensors and Actuators A 107 (2003) 209-218. An earlydisclosure of the use of magnetic labels to detect target molecules isto be found in Baselt (U.S. Pat. No. 5,981,297). Baselt describes asystem for binding target molecules to recognition agents that arethemselves covalently bound to the surface of a magnetic field sensor.The target molecules, as well as non-target molecules, are covalentlybound to magnetizable particles. The magnetizable particles arepreferably superparamagnetic iron-oxide impregnated polymer beads andthe sensor is a magnetoresistive material. The detector can indicate thepresence or absence of a target molecule while molecules that do notbind to the recognition agents (non-target molecules) are removed fromthe system by the application of a magnetic field.

A particularly detailed discussion of the detection scheme of the methodis provided by Tondra (U.S. Pat. No. 6,875,621). Tondra teaches aferromagnetic thin-film based GMR magnetic field sensor for detectingthe presence of selected molecular species. Tondra also teaches methodsfor enhancing the sensitivity of GMR sensor arrays that include the useof bridge circuits and series connections of multiple sensor stripes.Tondra teaches the use of paramagnetic beads that have very littleintrinsic magnetic field and are magnetized by an external source afterthe target molecules have been captured.

Coehoorn et al. (US Pub. Pat. Appl. 2005/0087000) teaches a system thatis similar to that of Tondra (above), in which magnetic nanoparticlesare bound to target molecules and wherein the width and lengthdimensions of the magnetic sensor elements are a factor of 100 or morelarger than the magnetic nanoparticles.

Prinz et al. (U.S. Pat. No. 6,844,202) teaches the use of a magneticsensing element in which a planar layer of electrically conductingferromagnetic material has an initial state in which the material has acircular magnetic moment. In other respects, the sensor of Prinzfulfills the basic steps of binding at its surface with target moleculesthat are part of a fluid test medium. Unlike the GMR devices disclosedby Tondra and Coehoorn above, the sensor of Prinz changes its magneticmoment from circular to radial under the influence of the fringingfields produced by the magnetized particles on the bound targetmolecules.

Gambino et al. (U.S. Pat. No. 6,775,109) teaches a magnetic field sensorthat incorporates a plurality of magnetic stripes spaced apart on thesurface of a substrate in a configuration wherein the stray magneticfields at the ends of the stripes are magnetostatically coupled and thestripes are magnetized in alternating directions.

Simmonds et al. (U.S. Pat. No. 6,437,563) teaches a method of detectingmagnetic particles by causing the magnetic fields of the particles tooscillate and then detecting the presence of the oscillating fields byinductively coupling them to coils. Thus, the sensor is not a GMR sensoras described above, but, nevertheless, is able to detect the presence ofsmall magnetic particles.

Finally, Sager et al. (U.S. Pat. No. 6,518,747) teaches the detection ofmagnetized particles by using Hall effect sensors.

The methods cited above that rely on the use of a GMR sensor, ratherthan methods such as inductive sensing or Hall effect sensing, will allbe adversely affected by the failure of the GMR sensor to maintain areproducible bias direction for its free layer magnetization. This lackof reproducibility is a result of magnetic hysteresis that occurswhenever the external magnetic fields being detected cause the magneticmoment of the sensor free layer to cycle about its bias direction. Inthe present use of the GMR sensor to detect the presence of extremelysmall magnetized particles, the external fields are small. Because ofthis, methods to fix the bias point of the sensor free layer cannot fixit too strongly as this would limit the ability of the free layermagnetic moment to respond to the very stimuli it is attempting tomeasure. It is, therefore, necessary to find a way of fixing the freelayer bias point while still allowing the magnetic moment sufficientfreedom of motion to detect even very small external magnetic fields.

SUMMARY OF THE INVENTION

A first object of this invention is to provide a method of determiningthe presence or absence of small magnetized particles.

A second object of this invention is to provide such a method thatdetects the aforementioned magnetized particles when they are bonded tochemical or biological molecules.

A third object of the present invention is to provide such a method thatuses the magnetoresistive properties of a GMR sensor to detect thepresence of a small magnetized particle.

A fourth object of the present invention is to provide a GMR sensor tobe used in detecting the presence of small magnetized particles whereinthe response of the sensor to external magnetic fields is not adverselyaffected by a non-reproducibility of its free layer bias point due tomagnetic hysteresis.

A fifth object of the present invention is to provide a GMR sensorhaving a high sensitivity and a free layer bias point that isreproducible.

The objects of the present invention will be achieved by a GMR sensordesign having the following characteristics, all of which areschematically illustrated in FIGS. 4 a and 4 b and will be discussedbelow in greater detail.

1. The sensor consists of multiple long stripes (only three being shownhere) of GMR films (1, 2, 3), electrically connected (500,600) inseries.2. The free and pinned layers of each sensor stripe are magneticallybiased, the biased magnetic moments being shown as single arrows, (11,22, 33), in the lengthwise direction.3. The sensor stripes are arranged in a serpentine configuration so thatadjacent stripes are substantially parallel to each other and have thebias positions of their magnetic moments oriented in paralleldirections.4. The spacing (44) between neighboring sensor stripes is much smallerthan the dimensions of the magnetic particles that they will bedetecting.5. The width of each stripe (800) is comparable to the dimensions of themagnetic particles being detected.6. The structure of each individual stripe is a capped lamination (seeFIG. 4 b) comprising a free layer (99) and a pinned layer (77) separatedby a metallic spacer layer (88), wherein the pinned layer can be asynthetic layer for enhanced pinning strength. The stripes aresurrounded by insulating layers (45).7. The magnetic anisotropy of each stripe is reduced by minimizing itsfree layer thickness and providing a minimal interlayer coupling betweenthe free and pinned layers.8. The free layer thickness is minimized, while not degrading thestripe's dR/R.9. The interlayer coupling is minimized by adjusting the thickness ofthe metallic layer separating the free and pinned layers.10. The film magnetostriction can be adjusted, in conjunction with anovercoat stress, to produce a net stress-induced anisotropy. With propercombination of these two stress factors, the easy axis of thestress-induced anisotropy can be oriented perpendicular to thelongitudinal direction of the stripe, so as to cancel out the free layershape anisotropy.

The characteristics enumerated above will produce a sensor having areproducible bias point while still retaining a free layer magnetizationthat is responsive to the effects of small external magnetic fields. Inparticular, by orienting the bias direction along the lengthwisedirection of the sensor stripe, the adverse hysteresis effects on astable bias point will be offset by the shape anisotropy produced by astripe shape that is longer than it is wide. By a combination ofmagnetostriction and stress-induced anisotropy that is perpendicular tothe shape anisotropy, however, the overall magnetization remainsresponsive and the sensor is sensitive to small external fields. Inaddition, by forming a narrow space (less than bead diameter) betweenadjacent stripes in an array, making the width of the stripes comparableto the dimensions of the bead and by orienting adjacent sensor stripesparallel to each other, the position of a magnetic particle is likely tooverlap two adjacent stripes, thereby, having its detectability enhancedby the series response of two stripes.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features, and advantages of the present invention areunderstood within the context of the Description of the PreferredEmbodiment as set forth below. The Description of the PreferredEmbodiment is understood within the context of the accompanying figures,wherein:

FIG. 1 (prior art) is a schematic representation of a magnetic beadbonded to a target molecule and the target molecule bonded to a receptorsite.

FIG. 2 a (prior art) is a schematic cross-sectional representation of aGMR sensor such as is positioned beneath the substrate of FIG. 1.

FIG. 2 b (prior art) is a schematic illustration of an overhead view ofthe sensor of FIG. 2 a, showing also the presence of an external fieldproduced by a magnetized particle.

FIG. 3 (prior art) is a schematic perspective representation of atypical biased GMR sensor stripe over which a magnetized particle ispositioned.

FIG. 4 a is a schematic overhead view of a sensor array formed of thesensor stripes of the present invention.

FIG. 4 b is a cross-sectional schematic view of one sensor stripe of thearray.

FIG. 5 is a schematic overhead view of two interconnected sensor stripesof the present invention showing the effects of a magnetized bead.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention are a GMR sensorstripe and an array of such GMR sensor stripes, capable of detecting thepresence of magnetic particles or beads, typically bonded to chemicalmolecules. The GMR stripe and the array of stripes, by virtue of theirformation, are not adversely affected by instability of a free layerbias point due to hysteresis. We use the term “stripe” to characterize aGMR sensor element and to emphasize the fact that it is deposited in theshape of a long, approximately rectangular strip or stripe. When used todetect magnetic particles bonded to target molecules (eg. in abio-chemical assay) the array is formed beneath a surface on which areaffixed bonding sites for target molecules. To perform the detectionprocess, the target molecules whose presence is to be detected, as wellas others that are not targets, are first magnetically tagged, by beingbonded to small magnetic particles or beads that are subsequentlymagnetized by an external magnetic field.

The advantages of the present invention reside in the fact that the biaspoint of the free and pinned layer magnetizations of each GMR sensorstripe in the array is oriented along the lengthwise direction of thestripe. The fact that the stripes are thin and longer then they arewide, provides a shape anisotropy that maintains a bias point in thelengthwise direction that is stable with respect to hysteresis effectsproduced by the cyclic motion of the free layer magnetic moment duringits use in detection processes. In order to ensure that the shapeanisotropy does not adversely affect the sensitivity of the sensor tosmall external fields that move the magnetization away from thelongitudinal bias direction, a compensating anisotropy is produced bycombining a stress induced anisotropy due to magnetostriction of thesensor magnetic layers with the stresses in the magnetic layers producedby tension or compression of the various surrounding sensor overlayersthat encapsulate the sensor. This combination of magnetostriction andcompressional or tensile overlayer stress can be adjusted to reduce theoverall magnetic anisotropy. Finally, the sensor free layer is made asthin as possible while not sacrificing the GMR ratio, dR/R, and theinterlayer coupling between the free and pinned layers is adjusted to besmaller than the magnetic anisotropies.

The sensor stripes produced by the methods of this invention are thenconnected in electrical series in a serpentine fashion that placesindividual stripes side-by-side in a parallel configuration, with anarrow space between adjacent stripes and with the bias directions oftheir magnetizations (i.e., their magnetic moments) parallel. To achievethis configuration, the individual stripes are placed side-by-side asdesired and then electrically connected between the aligned top andbottom edges of adjacent stripes with a conducting element to create acontinuous electrical circuit. Because the stripes are very narrowlyspaced (less than a bead diameter) and are very narrow themselves(approximately a bead diameter) there is a great likelihood thatindividual beads located above the stripes will straddle two adjacentstripes, thereby, enhancing the response of the array.

Because the methods of forming the binding surface, the nature andformation method of the binding sites and the means of attaching themagnetic beads to the target molecules are all well known in the art(see the above cited journal article and the prior art patents), thedetailed description of the invention that now follows will berestricted to the construction of the sensor stripes and the arrayconfiguration.

Referring now to FIG. 4 a, there is shown a schematic overhead view of asmall array of GMR stripes or, equivalently, what could be a segment ofa larger array, in which there are three electrically connected GMRsensor stripes of the present invention, denoted for reference purposesas stripes 1, 2 and 3. These stripes are of generally rectangular shape,having parallel lateral edges (101), (202), (303) of length betweenapproximately 10 microns and 200 microns and parallel transverse edges(111), (222), (333) of width between approximately 1 micron and 5microns. The stripes are connected in electrical series in anelectrically conductive continuous serpentine configuration that alignssuccessive stripes adjacent to each other with their magnetic moments,when in a quiescent state, oriented in parallel (arrows (11), (22),(33)). The separation (44) between adjacent stripes (filled by thesurrounding layers of insulation (45)) is less than the diameter of themagnetic particles to be detected, which are typically betweenapproximately 0.2 microns and 1 micron. As can be seen in the figure,the co-linear upper transverse edges (111), (222) of stripes 1 and 2,are electrically connected with a conducting element (500), as are thelower transverse edges (222), (333) of stripes 2 and 3 (600). The freelower edge of stripe 1 (111) and the free upper edge of stripe 3 (333)are each conductively connected to terminal connectors (550) for thepurpose of engaging the array within an external circuit (not shown). Ifthe three stripes are part of a larger array, the terminal connectorswould be absent and connections to other GMR stripes would be made. Ascan be envisioned, if the array consisted of M stripes, the connectionswould proceed, pairwise, in like fashion, with end stripes 1 and M beingconnected to terminals. It is understood that the array of FIG. 4 a willbe encapsulated within surrounding layers of insulation (45).

The dimensional difference between the length and width of each sensorstripe gives the stripe a shape asymmetry that produces a magneticanisotropy along the lengthwise dimension. This anisotropy assists inmaintaining the bias point (the magnetic moment under quiescentconditions) of the free layer when that bias point is also in thelengthwise direction as shown in FIG. 4 a. However, the magneticanisotropy cannot be too great or it will impede the variations inmagnetic moment of the free layer under the action of external magneticfields. Thus, some degree of additional magnetic anisotropy must beincorporated into the sensor stripe in order to produce the requiredsensor sensitivity. This will now be discussed with reference to FIG. 4b.

Referring to FIG. 4 b there is shown a cross-sectional view of a singleGMR stripe, such as either of the three stripes in FIG. 4 a,illustrating, schematically, the preferred sequence of layers that formthe GMR sensor stripe. Looking from the bottom up, there is shown asubstrate (55), which can be a layer of oxide, a pinning layer (66),which can be a single layer of antiferromagnetic material, a pinnedlayer (77) which can be either a single layer of ferromagnetic material,such as CoFe or NiFe, formed to a thickness between approximately 10 and100 angstroms or a laminated synthetic antiferromagnetic layer formed oftwo such ferromagnetic layers coupled by a non-magnetic coupling layer,a spacer layer (88) of a non-magnetic, electrically conducting materialsuch as Cu, formed to a thickness between approximately 10 and 20angstroms, a free layer (99) formed of a ferromagnetic material such asCoFe or NiFe, to a thickness between approximately 10 and 100 angstroms,and an overlayer (100) or capping layer to protect the sensor structure.The overlayer can be a portion of the surrounding insulating layers,which are formed of oxides or nitrides of Si or it can be a portion ofthe layer that supports the bonding sites for the magnetically taggedparticles, the supporting layers being typically formed of similarinsulating materials. After the sensor stripe is fabricated, the pinnedand free layers are annealed to set their magnetic moment directions(i.e, their magnetizations) along the lengthwise dimension of the stripeas shown here as (777) and (999) also in FIG. 4 a (as (11), (22) and(33)) so that the bias point (direction of the magnetic moment when thestripe is quiescent, i.e. is not acted on by external fields) is alongthe lengthwise direction of the stripe. It is further noted that thestripe is surrounded by layers of insulation (45), such as alumina oroxides or nitrides of silicon formed to thicknesses betweenapproximately 1000 angstroms and 2 microns, to isolate it electricallyfrom neighboring circuit elements (not shown) and that such insulatingmaterial will contribute to stresses exerted on the stripe.

By adjusting the spacer layer (88) the interlayer coupling between thefree (99) and pinned (77) layers can be reduced so that the variation ofthe free layer magnetization in response to small external fieldsproduces the required response of the sensor. Further, the free layeritself must be made as thin as possible, without sacrificing the dR/R ofthe sensor (the measure of its sensitivity), so that the free layer isresponsive to small external fields. In addition, as is known in theart, the ferromagnetic layers exhibit the phenomenon ofmagnetostriction, which is typically defined in terms of a coefficientof magnetostriction. For example, NiFe alloy has a coefficient ofmagnetostriction that approaches zero at a composition of about 19% Fe.The coefficient becomes negative with less Fe and positive with more Fe.A thin layer (such as is formed herein) of positive coefficient ofmagnetostriction will exhibit a magnetic anisotropy in a direction oftensile stress on the layer. Likewise, a film having a negativecoefficient of magnetostriction will exhibit a magnetic anisotropy in adirection of compressive stress on the layer. As the GMR sensor is ametallic stripe (as shown in FIG. 4 b) encapsulated in surroundinginsulation layers from above and below (and possibly including thesubstrate itself), it will generally be under anisotropic compressive ortensile stress that is substantially within the plane of the sensorlayers. The magnitude of this stress will depend on the material formingthe surrounding insulation layers and specific processes involved intheir fabrication. The magnetostriction coefficient of the GMR sensorcan be adjusted by its composition to give a magnetostrictioncoefficient that, when combined with the anisotropic stress of thesurroundings, will result in a stress induced magnetic anisotropy thatis perpendicular to the lengthwise direction of the stripe. For example,if the anisotropic stress of the GMR sensor is tensile in the lengthwisedirection, the magnetically free layer magnetostriction coefficient isadjusted to be slightly negative, so that the stress induced magneticanisotropy will be perpendicular to the lengthwise direction of thestripe while the magnitude is small enough so that the net anisotropy isstill in the lengthwise direction.

Referring now to FIG. 5, there is shown, schematically, just stripes 1and 2 of the array in FIG. 4 a. A magnetized bead (not shown) is locatedabove the separation between the stripes and produces two lobes (1001)and (2002) defining equal-strength field lines of its magnetic field.The field vectors are directed as shown by the enclosed arrows, withinthe plane of the stripes. It can be seen that the parallel configurationof the adjacent stripes 1 and 2 and the new orientation of theirmagnetic moments (11) and (22) caused by the field of the magnetizedbead, combined with the narrow separation between stripes, thenarrowness of each stripe and the series connection of the stripes,produces a significant enhancement of the sensor's response. The maximumresponse of the sensor array to the presence of a magnetized particleoccurs when the particle is over the separation between adjacentstripes, as shown in this figure. In that position, each of the twolobes causes a strong deflection of the magnetic moments of therespective stripes. Because of the series connection of the two stripes,the dR/R of each stripe effectively add to produce a doubling of thevoltage drop across the array. If the magnetic bead is not preciselyover the separation between stripes, the narrow width of each stripestill ensures that the magnetic field of the bead impinges on more thanone stripe and enhances the response of the array.

As is finally understood by a person skilled in the art, the preferredembodiments of the present invention are illustrative of the presentinvention rather than limiting of the present invention. Revisions andmodifications may be made to methods, materials, structures anddimensions employed in forming and providing a GMR sensor stripe arraywith a stable free layer bias point, while still forming and providingsuch an array and its method of formation in accord with the spirit andscope of the present invention as defined by the appended claims.

1. A linear array of GMR sensor stripes electrically connected in aserpentine configuration for the purpose of detecting small magneticparticles, comprising: a substrate; an integer plurality of Msubstantially identical horizontally planar GMR stripes formed on saidsubstrate, denoted successively 1 to M, each stripe having a rectangularshape with a length dimension greater than a width dimension and amagnetic bias direction along said length dimension, and wherein thewidth dimension of each said stripe has a top edge and a bottom edge andwherein said stripes are placed side by side in a common horizontalplane and aligned with their top and bottom edges separately co-linearand their lengthwise sides adjacent to each other, parallel to eachother and separated from each other by a separation distance and whereinthe top edges of adjacent stripes denoted 1 and 2 are electricallyconnected by a conducting element and wherein the bottom edges ofadjacent stripes denoted 2 and 3 are electrically connected by aconducting element and wherein the remaining stripes from 3 to M aresimilarly connected to form a linear array with a continuouselectrically conducting pathway from an unconnected bottom edge ofstripe 1 to an unconnected edge of stripe M.
 2. The array of claim 1wherein said separation distance is less than the diameter of the smallmagnetic particles to be detected.
 3. The array of claim 1 wherein thewidth of each said stripe is approximately the diameter of the smallmagnetic particles to be detected.
 4. The array of claim 1 furtherincluding an encapsulating surrounding layer of insulating material,said insulating material placing said stripes in a condition ofanisotropic compressive or tensile stress.
 5. A method of forming anarray of GMR sensor stripes whereby such array can detect the presenceof small magnetized particles and whereby the individual GMR stripesforming said array are not be adversely affected by magnetic hysteresis,comprising: providing a substrate; forming on said substrate a pluralityof substantially identical, planar, horizontal, rectangular GMR stripes,each stripe having parallel lateral edges in the lengthwise directionand parallel transverse edges in the widthwise direction, wherein thelength of said lateral edges is greater than the length of saidtransverse edges; placing said GMR stripes so that proximal lateraledges of adjacent stripes are parallel and separated by a separationdistance and aligned along said lateral edges so that correspondingtransverse edges are co-linear; then connecting transverse edges ofadjacent stripes with an electrically conducting element so that saidplurality of GMR stripes forms a continuous linearly connectedelectrical circuit; and encapsulating said stripes within a surroundinglayer of insulating material.
 6. The method of claim 5 wherein each GMRstripe is formed by a method comprising: providing a substrate; formingon the substrate a magnetically pinning layer; forming on said pinninglayer a magnetically pinned layer forming on said pinned layer aconducting, non-magnetic spacer layer; forming on said spacer layer amagnetically free layer; forming on said magnetically free layer acapping layer; wherein all said layers are substantially rectangular andlonger in a lengthwise direction than in a transverse direction; andmagnetizing said pinned layer and said free layer along a lengthwisedirection.
 7. The method of claim 6 wherein said magnetically free layeris formed of a Co, Fe, Ni ferromagnetic alloy to a thickness betweenapproximately 10 angstroms and 100 angstroms.
 8. The method of claim 6wherein said spacer layer is formed of Cu to a thickness betweenapproximately 10 angstroms and 20 angstroms.
 9. The method of claim 5wherein said encapsulating layer is formed of the insulating materialalumina, silicon oxide or silicon nitride, to a thickness betweenapproximately 1000 angstroms and 2 microns.